The present invention relates generally to optical systems and exposure apparatuses, and more particularly to a method and apparatus for retaining a mirror used in an exposure apparatus. The present invention is suitable, for example, for an illumination optical system and projection exposure apparatus using an extreme ultraviolet (“EUV”) region having a wavelength of 200 nm to 10 nm or an X-ray region.
Reduction projection exposures using ultraviolet have been conventionally employed to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology. The critical dimension to be transferred by the reduction projection exposure is proportionate to a wavelength of light used for transfer, and inversely proportionate to the numerical aperture (“NA”) of a projection optical system. In order to transfer a finer circuit pattern, a shorter wavelength of used ultraviolet (“UV”) light has been promoted from an ultra-high pressure mercury lamp i-line with a wavelength of about 365 nm to KrF excimer laser with a wavelength of about 248 nm and ArF excimer laser with a wavelength of about 193 nm.
However, the lithography using the UV light has the limit to satisfy rapidly promoting fine processing of a semiconductor device, and a reduction projection exposure apparatus using EUV light with a wavelength of about 10 to 15 nm much shorter than that of the ultraviolet has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The EUV light source uses, for example, a laser plasma light source. The laser plasma light source irradiates a highly intensified pulse laser beam to a target in a vacuum chamber, and generates high-temperature plasma, emitting EUV light with a wavelength of about 13 nm. The target uses a metallic thin film, inert gas, droplet, etc., and is supplied to the vacuum chamber by such means as a gas jet. In order to raise an average intensity of the emitted EUV light, the pulse laser preferably has a higher repetitive frequency, and is usually driven by the repetitive frequency of several kHz.
Absorption in an object of EUV light region is so large that a deflection optical system that uses a lens may lower throughput, while it is usually used for visual light and UV light. Therefore, exposure apparatuses that use EUV light usually include a cataoptric optical system. For example, isotropically emitted EUV light from the laser plasma is then condensed by a first condenser mirror in an illumination optical system, and emitted to the next mirror to illuminate a mask.
The laser plasma light source generates not only the EUV light, but also flying particles called debris, which causes contamination, damages and lowered reflectance of an optical element. While some methods have been disclosed, for example, in Japanese Patent Application Publication No. 2000-349009, which prevent debris from reaching an optical element from the target, there has not been proposed a method for effectively preventing debris from reaching the first stage mirror, particularly close to the target, in the illumination optical system. As a result, the debris adheres to a surface on the first mirror and lowers its reflectance over exposure time. The first mirror should thus be replaced regularly when the reflectance lowers down to a certain level. A method for facilitating an exchange and maintenance of the mirror has been proposed, for example, in Japanese Patent Applications Publications Nos. 5-100096 and 7-174896 (corresponding to U.S. Pat. Nos. 5,448,612 and 5,572,563).
A description will be given of a conventional mirror replacement method proposed in Japanese Patent Application Publication No. 5-100096, with reference to FIGS. 9 and 10. Here, FIG. 9 is a schematic partial section of a vacuum chamber that accommodates an illumination system of an exposure apparatus. FIG. 10 is a flowchart for explaining a conventional mirror replacement method. A first mirror 4 is retained by a mirror holder 2 fixed in a vacuum chamber 1 that accommodates an illumination system of an exposure apparatus. The vacuum chamber 1 has an openable door 6. A water cooled tube 8 is connected to a mirror holder 2 and cools it. The water cooled tube 8 is connected to the door 6, and receives cooling water from the outside of the door 6.
In exchanging the mirror 4, the vacuum chamber 1 is returned to the atmospheric pressure (step 1002), the door 6 is opened (step 1004), and the water cooled tube 8 is dismounted from the door 6 (step 1006). Then, a hand is inserted from the door 6, and the mirror 4 is dismounted from the mirror holder 2 (step 1008), a new mirror 4 is mounted onto the mirror holder 2 (step 1010) and its reflective surface is optically and mechanically positioned (step 1012). Then, the water cooled tube 8 is attached to the door 6 (step 1012), and the door 6 is shut (step 1014), followed by the step of drawing a vacuum (step 1016). Thus, the conventional exchange of the mirror 4 requires a large maintenance space in the exposure apparatus and a long maintenance time, disadvantageously lowering exposure throughput and contaminating mirrors, such as an illumination optical system, and the chamber 1 due to a long opening time of the vacuum chamber 1.
Japanese Patent Application Publication No. 7-174896 discloses a mirror retaining method that uses part of a mirror for a partition of the vacuum chamber. This method may shorten an exchange time, because when the mirror is attached to a vacuum chamber, a mirror itself is simultaneously positioned. However, actually, the vacuum chamber is likely to deform and the mirror also undesirably deforms along with a deformation of a wall surface of the vacuum chamber after the mirror is positioned by attaching it to the chamber.